1. Field of the Invention
The present invention relates to magnetic resonance imaging systems, more particularly, to methods for improving transmit spin excitation.
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) has proven to be an enormously useful technology both for the detection and diagnosis of human disease as well as for research into the understanding of basic human and animal physiology.
For the acquisition of image data from a nuclear magnetic resonance (NMR) signal, four separate components are required, as shown in FIG. 1. First, a static magnetic field must be generated by a constant field magnet 12, generally of the superconducting type. Pursuant to quantum mechanics, the presence of the static magnetic field causes in a subject an energy difference between nuclear spins aligned with and against this static magnetic field. The magnitude of the energy difference depends on a variety of factors, including strength of the magnetic field, size of the magnetic moments of individual atomic nuclei, and temperature. In general, a majority of the nuclear spins will align with the static magnetic field and a higher energy minority of the nuclear spins will align against it. When exposed to an oscillating magnetic field of proper frequency, such as is generated by an alternating current in a radio frequency (RF) transmit coil structure 14, some of the lower energy spins aligned with the static magnetic field will be excited to the higher energy state of being aligned against the field. Once the applied transmit RF magnetic field is removed, these excited spins will decay to the lower energy state of alignment with the static magnetic field. During the decay, these spins will generate their own RF magnetic field, which can be electronically detected by the same or a different RF coil structure 16 and thereby be characterized. In order to determine spatial information about the quantity and properties of the atomic nuclei of the subject, a second set of gradient coils 18 are used to perturb the static magnetic field. By generating magnetic field gradients, current in this separate set of coils 18 spatially changes the oscillation frequency of the nuclear spins by changing the frequency of the nuclear magnetic resonance (NMR) oscillation at appropriate times during transmit and receive, and spatial information regarding the nuclear spins can be decoded and converted into an image. The generation of the NMR signal in the transmit coil structure 14, the reception of the NMR signal by the receive coil structure 16, and the currents in the gradient coils are controlled by a computer system 20 which processes the information obtained and displays it on a computer screen or printed film for human interpretation.
Current technology high frequency volume coils for magnetic resonance imaging (MRI) brain and body imaging have RF magnetic field inhomogeneities caused by wavelength effects (or “dielectric resonance”) and conductive shielding effects inside human tissue. For image reception, these field inhomogeneities are of less concern as they can be adjusted by spatially correcting the image intensity variations caused by the inhomogeneous reception fields after image processing. However, inhomogeneities in the transmit field cause non-uniform spin excitation which leads to significant reductions in signal-to-noise ratio (SNR) and image contrast. These effects can be seen at 3 Tesla (T) field strengths for human brain and body imaging and become pronounced at 4 T and higher field strengths.
Transmit systems such as described in U.S. Pat. No. 6,501,274 attempt to correct the field distortions caused by dielectric wave effects by independently controlling individual elements in a volume coil, that is, by controlling the spatial magnetic field profile (which does not change either during a given spin excitation or between successive spin excitations). While this can result in improvement in transmit field homogeneity, Maxwell's equations limit the achievable reduction in inhomogeneities because field patterns that correct for inhomogeneities in one area worsen the inhomogeneities in other areas. At high frequencies, a homogeneous magnetic field is not a solution to Maxwell's equation in a dielectric body.
Examples of field inhomogeneities are shown in FIGS. 2 and 3, which show magnetic field profiles from an eight element volume coil driven with quadrature excitation at 128 MHz (3 T) and 300 MHz (7 T), respectively, using a finite difference, time domain (FDTD) method calculation with a 5 mm head/body mesh. The images are of an axial field slice through the mid portion of a human brain. As is typically found at ultrahigh frequencies, fields from a volume coil are more intense at the brain center than at the edges. In particular, there is a region in the parietal and temporal lobes which has particularly low field intensity.